Educational

Magnetic Properties of Materials

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by

Saim Khalid
in

Magnetism is a fundamental property of matter associated with moving charges and electron spins. The study of magnetic properties of materials is critical in physics, engineering, and technology. Understanding how materials respond to magnetic fields allows scientists and engineers to design devices such as transformers, magnetic storage devices, motors, and sensors.

1. Introduction to Magnetism

Magnetism originates at the atomic level due to:

  1. Electron motion around the nucleus (orbital motion),
  2. Electron spin, which generates a magnetic moment.

A material’s response to an external magnetic field depends on how these atomic magnetic moments align within the material.

Magnetic behavior is generally studied using magnetization (M), which is the magnetic moment per unit volume, and magnetic susceptibility (χ), which indicates how easily a material is magnetized. M=χHM = \chi HM=χH

Where HHH is the applied magnetic field strength.

2. Magnetic Moment

The magnetic moment of an atom arises due to:

  • Orbital motion of electrons: Electrons revolving around the nucleus generate a current loop, producing a magnetic moment.
  • Electron spin: Each electron has an intrinsic spin magnetic moment, contributing to the atom’s net magnetic moment.

The net magnetic moment of a material determines its overall magnetic behavior.

3. Classification of Magnetic Materials

Materials can be classified based on their response to external magnetic fields:

  1. Diamagnetic Materials
  2. Paramagnetic Materials
  3. Ferromagnetic Materials
  4. Antiferromagnetic Materials
  5. Ferrimagnetic Materials

Each type has distinct properties and applications.

3.1. Diamagnetic Materials

Characteristics:

  • Weakly repelled by magnetic fields.
  • Magnetic susceptibility (χχχ) is negative.
  • No permanent magnetic moment.
  • Examples: Bismuth, Copper, Gold, Silver, Water.

Origin: Diamagnetism arises from the induced magnetic moments due to Lenz’s Law when an external field is applied. Electrons adjust their orbital motion to oppose the applied field.

Applications:

  • Magnetic levitation (using diamagnetic repulsion).
  • Shielding sensitive devices from magnetic fields.

3.2. Paramagnetic Materials

Characteristics:

  • Weakly attracted by magnetic fields.
  • Magnetic susceptibility (χχχ) is positive but small.
  • Do not retain magnetization after removing the field.
  • Examples: Aluminum, Platinum, Oxygen (O₂ gas).

Origin: Caused by unpaired electron spins that tend to align with an external field, but thermal agitation prevents permanent magnetization.

Curie Law: Paramagnetic susceptibility varies with temperature: χ=CT\chi = \frac{C}{T}χ=TC​

Where CCC is the Curie constant and TTT is the absolute temperature.

Applications:

  • Magnetic resonance imaging (MRI) contrast agents.
  • Materials in chemical analysis.

3.3. Ferromagnetic Materials

Characteristics:

  • Strongly attracted to magnetic fields.
  • Exhibit spontaneous magnetization even without an external field.
  • Can retain magnetization (hysteresis) after the field is removed.
  • Examples: Iron, Cobalt, Nickel, Gadolinium.

Origin: Arises due to exchange interaction, which aligns atomic magnetic moments in domains.

Key Concepts:

  • Magnetic Domains: Regions where magnetic moments are aligned. Domains randomly oriented in unmagnetized material.
  • Curie Temperature (TcT_cTc​): Temperature above which ferromagnetic material becomes paramagnetic.

Applications:

  • Permanent magnets.
  • Transformers, motors, generators.
  • Magnetic storage devices (hard disks).

3.4. Antiferromagnetic Materials

Characteristics:

  • Neighboring atomic moments align in opposite directions.
  • Net magnetization is zero.
  • Weakly responds to magnetic fields.
  • Examples: Manganese oxide (MnO), Iron oxide (FeO).

Origin: Exchange interaction causes anti-parallel alignment of spins.

Neel Temperature (TNT_NTN​): Temperature above which antiferromagnetic materials become paramagnetic.

Applications:

  • Spintronic devices.
  • Magnetic sensors.

3.5. Ferrimagnetic Materials

Characteristics:

  • Similar to antiferromagnetic but unequal opposing moments result in net magnetization.
  • Examples: Magnetite (Fe₃O₄), Ferrites.

Origin: Magnetic moments of ions are antiparallel but unequal, causing a net magnetic moment.

Applications:

  • Ferrites used in transformers, inductors, and microwave devices.
  • Magnetic recording media.

4. Magnetic Hysteresis

Hysteresis refers to the lag between magnetization and applied magnetic field in ferromagnetic materials. A hysteresis loop graph shows:

  • Coercivity (Hc): Field needed to reduce magnetization to zero.
  • Remanence (Mr): Residual magnetization after removing the applied field.
  • Saturation (Ms): Maximum magnetization achievable.

Applications:

  • Designing magnetic cores.
  • Permanent magnets.
  • Magnetic memory devices.

5. Permeability of Materials

Permeability (μ) indicates how easily a material can be magnetized. μ=BH\mu = \frac{B}{H}μ=HB​

Where BBB is the magnetic flux density and HHH is the applied magnetic field.

  • Diamagnetic: μ < μ₀ (slightly less than vacuum permeability)
  • Paramagnetic: μ > μ₀ (slightly more than vacuum)
  • Ferromagnetic: μ >> μ₀ (can be hundreds to thousands times larger)

6. Temperature Effects

Magnetic properties depend on temperature:

  • Curie Temperature (Tc): Above this, ferromagnetic → paramagnetic.
  • Neel Temperature (TN): Above this, antiferromagnetic → paramagnetic.
  • Diamagnetic and paramagnetic materials: Susceptibility decreases with increasing temperature (Curie Law for paramagnetics).

7. Quantum Mechanical Origin of Magnetism

Magnetism at the atomic level arises from quantum mechanics:

  1. Electron Spin: Intrinsic property; electrons have spin angular momentum.
  2. Pauli Exclusion Principle: Determines electron arrangement in orbitals, affecting net magnetic moment.
  3. Exchange Interaction: Quantum effect causing alignment of neighboring spins, responsible for ferromagnetism and antiferromagnetism.

8. Measurement of Magnetic Properties

  1. Vibrating Sample Magnetometer (VSM): Measures magnetization of samples.
  2. SQUID (Superconducting Quantum Interference Device): Extremely sensitive magnetometer.
  3. Hysteresisgraph: Measures hysteresis loop and related parameters.
  4. Magnetic Susceptibility Balance: Determines susceptibility of diamagnetic and paramagnetic materials.

9. Magnetic Anisotropy

Magnetic properties often depend on direction within the material:

  • Crystalline Anisotropy: Magnetization is easier along certain crystal axes.
  • Shape Anisotropy: Depends on geometry of the sample.
  • Stress-Induced Anisotropy: Mechanical stresses affect domain orientation.

Applications: Design of magnetic recording media, permanent magnets, and sensors.

10. Soft vs. Hard Magnetic Materials

  • Soft Magnetic Materials: Easily magnetized and demagnetized; low coercivity. Used in transformers and inductors.
  • Hard Magnetic Materials: Retain magnetization; high coercivity. Used in permanent magnets.

Examples:

  • Soft: Silicon steel, soft iron.
  • Hard: Alnico, NdFeB magnets.

11. Applications of Magnetic Materials

  1. Electrical Engineering:
    • Transformers, motors, generators.
    • Inductors and chokes.
  2. Electronics:
    • Magnetic sensors (Hall effect sensors).
    • Magnetic storage (hard disks, tapes).
  3. Medical Field:
    • MRI imaging using paramagnetic contrast agents.
  4. Magnetic Levitation:
    • Maglev trains use diamagnetic and superconducting magnets.
  5. Data Storage:
    • Hard drives and magnetic tapes use ferromagnetic materials for recording.

12. Advanced Magnetic Materials

  • Ferrites: High resistivity, used in high-frequency transformers.
  • Superparamagnetic materials: Nanoparticles exhibit paramagnetism but with high magnetic moments.
  • Spintronic materials: Exploit electron spin for next-gen electronics.

13. Magnetic Domains

  • Ferromagnetic materials are divided into domains where magnetic moments are aligned.
  • Domain walls separate regions with different orientations.
  • External fields can shift domain boundaries, increasing net magnetization.

Applications: Understanding domain behavior is crucial for minimizing energy losses in transformers.

14. Energy Considerations

  • Magnetic Energy Density: Energy stored in magnetic field:

u=12BHu = \frac{1}{2} B Hu=21​BH

  • Magnetic materials store and release energy efficiently, important for inductors and transformers.

15. Magnetic Losses

  • Hysteresis Loss: Energy lost in magnetizing and demagnetizing cycles.
  • Eddy Current Loss: Circulating currents induced in conductive materials by changing magnetic fields.

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